1. Technical Field
This disclosure relates to heat-sensitive adhesive materials, containing a heat-sensitive adhesive layer, that exhibit superior adhesive strength with adherends and activate thermally even with lower energy, in which the heat-sensitive adhesive layer is non pressure-sensitive at ordinary temperatures and pressure-sensitive at higher temperatures, and also the pressure-sensitiveness can be significantly maintained after development thereof.
2. Description of the Related Art
Pressure-sensitive label sheets have been spreading their applications, for example, in the fields of price-display labels, commodity-display or bar-code labels, quality-display labels, weight-display labels, and advertising labels or stickers. Further, various methods for recording labels have been developed such as inkjet recording methods, hot-melt recording methods, and pressure-sensitive recording methods.
Such pressure-sensitive label sheets typically have a construction that a pressure-sensitive layer and a peeling paper are laminated to a surface opposite to an information recording surface, which construction is extremely popular by virtue that simple operation of pressing after removing a peeling paper may conveniently provide an adhered condition. The peeling papers generally are not recovered and reused, namely are discarded in most cases. Recently, pressure-sensitive label sheets, which comprise a heat-sensitive adhesive layer and no need of the peeing paper owing to no pressure-sensitive at ordinary temperatures, have been attracting attentions for use as thermosensitive materials (see Japanese Utility Model Application Laid-Open No. 06-25869).
The heat-sensitive adhesive layers of such thermosensitive label sheets are comprised of a thermoplastic resin, hot-melt material, and optional tackifier, for example (see “Adhesive Handbook”, 12 th edition, pp 131-135, 1980, by Kobunshi Kankokai).
However, the heat-sensitive adhesive layer of such thermosensitive label sheet suffers from gradual decrease of the pressure-sensitive adhesive strength after the development of pressure-sensitiveness, and excessively high energy is required for thermal activation.
In order to solve the deficiencies, a thermal insulation layer is proposed that contains hallow plastic particles and a water-soluble binder and is present between a support and a heat-sensitive adhesive layer so as to lower the thermal energy i.e. to raise the sensitivity during the thermal activation (see Japanese Patent (JP-B) No. 2683733 and Japanese Patent Application Laid-Open (JP-A) No. 10-152660). The proposal may provide a successful effect to decrease thermal energy for activating thermally the heat-sensitive adhesive layer. However, the proposal suffers from insufficient pressure-sensitive adhesive strength with respect to rough adherends such as cardboards or polyolefin wraps since a water-soluble binder is utilized that is substantially no pressure-sensitive at ordinary temperatures. Moreover, the proposal also suffers from gradual decrease of the pressure-sensitive adhesive strength after the development of pressure-sensitiveness. As such, investigation and improvement are still required currently.
Films and synthetic papers have been expanding their market for use as supports from the viewpoints of waterproofness, label strength, and smoothness. However, the market of the heat-sensitive adhesive materials does not follow the expanding market of the supports of films and synthetic papers. The reasons are considered as follows.
(i) The supports of films or synthetic papers tend to refuse permeation of the heat-sensitive adhesive layer, often resulting in disadvantages such as repelling, pin holes, and nonuniformity.
(ii) The supports of films or synthetic papers typically represent a lower anchor effect with an active layer, thus the active layer tends to fall off during thermal activation.
(iii) The supports of films or synthetic papers tend to shrink and wrinkle itself during thermal activation.
(iv) The supports of films or synthetic papers exhibit higher strength and is less likely to break, thus tends to leave only the heat-sensitive adhesive layer on commodities when labels on commodities are to be peeled off, resulting in inferior appearance.
The item (i) described above may possibly be improved by optimizing the amount or species of surfactant or dispersant added to the heat-sensitive adhesive layer. However, the items (ii) to (iv) have not been solved yet since the problems issue inherently from thermosensitive materials and synthetic papers.
Blocking is one of serious problems in terms of previous thermosensitive materials, which is a phenomenon to generate pressure-sensitivity even when the pressure-sensitivity is undesired still. The blocking is often induced when the thermosensitive material is exposed at temperatures higher than ordinary temperatures for a long time. When the blocking is induced either in a condition of a roll or in a condition of cut and stacked sheets, the heat-sensitive adhesive layer adheres to the facing surface, consequently, the paper-feed comes to troublesome, and also the printing tends to be adversely affected.
With respect to the methods or means to prevent the blocking, disclosed are a method to compound a lubricant wax into heat-sensitive adhesive materials (see Japanese Patent Application Publication (JP-A) No. 62-21835), a method to prevent the blocking by way of protecting a surface of hot-melt material with an inorganic compound or colloid particles and suppressing the softening of the hot-melt material (see JP-A Nos. 06-57223, 06-100847, and 06-100848), and the like. However, the compounding of waxes leads to not only insufficient effect to prevent blocking but also adverse affect to decrease the pressure sensitivity. When the surface of hot-melt material is protected with an inorganic compound or colloid particles, such disadvantages tend to appear that melting or diffusing of the hot-melt materials requires longer period, the pressure sensitivity hardly generates in the heat-sensitive adhesive material, and adhesive property is inferior, thus resulting in practical difficulties. In addition, the blocking may be avoided by use of a plasticizer having a high melting point; however, adverse effects such as remarkable drop of adhesive strength generate, thus the transportation and storage are practically carried out at lower temperatures at which the blocking is far from the occurrences.
Further, printers are demanded for improving safety, power saving, and compactification. For example, JP-A Nos. 11-79152, 11-65451, 10-35126, 11-157141, 11-311945, 2001-303036, and 2001-48139 illustrate thermal activation methods or activation units by means of thermal heads.
Thermal heads for activation may reduce energy consumption at the thermal activation, and also the power saving and compactification may be improved. However, the thermal head utilized for an activation means inevitably results in serious problems such as decrease of adhesive strength and short life of the thermal head, since the activation is performed through heating and contacting the thermal head with the heat-sensitive adhesive layer, a large amount of adhesive dregs or debris deposits on a resistor of the thermal head, and the thermal energy cannot be effectively supplied to the heat-sensitive adhesive layer.
In order to solve these problems, JP-A No. 11-79152 described above proposes a thermal head on which a releasant layer is disposed, and JP-A No. 11-65451 described above proposes a thin sheet disposed between the thermal head and the heat-sensitive adhesive layer. However, these proposals suffer from insufficient adhesive strength since the thermal conductivity decreases and thermal energy cannot be supplied sufficiently to the thermal activation layer; the cost for producing the thermal heads comes to higher; thus these proposals have not been realized in the current market.